Control devices for piezoelectric elements are used in fuel injection systems of a motor vehicle, for example. In this connection, piezoelectric elements of this type are used as an actuator to which a control current may be applied. For the control of such an actuator, the voltage applied to the actuator must be monitored with high precision.
The operativeness of injection systems for internal combustion engines is based on an exact control of the fuel-injection actuators that are designed as piezo actuators. In DE 10 2004 037 720 A1, a control circuit for an actuator is discussed, piezoelectric elements being controlled that move a valve needle of an injection valve, for example, in order to bring about a fuel injection into a combustion chamber of the internal combustion engine.
The control circuit discussed in DE 1 037 720 A1 is designed such that interference currents, which occur as a result of lead inductances of the piezoelectric elements at specified current characteristics, are avoided to the greatest possible extent.
German patent document DE 10 2004 058 671 A1 discusses an additional electric circuit for controlling a piezoelectric element in particular of a fuel injection system of a motor vehicle. In this connection, two transistors, which are connected in series and are controllable using one pulse, are provided whose shared connecting point is coupled to the piezoelectric element and of which one is provided for discharging the piezoelectric element. Indeed, in the case of an error, a fast discharge of the piezoelectric element may be ensured; however, disadvantageously, using the described system it is not possible to monitor a voltage at the piezoelectric element reliably and without ground potential.
In order to be able to introduce fuel into a combustion chamber using a piezoelectric element, the piezoelectric element must be charged to a voltage of up to 200 V. A charging/discharging of the piezoelectric element (the actuator unit) is done using a pulsed current. In customary common rail piezo diesel systems, one pole of the piezoelectric element is connected to the ground potential. Thus, to measure the actuator voltage, potential at the other end of the piezoelectric element may be detected and determined relative to ground.
However, in the further refinement of piezoelectric elements for highly precise fuel injection, none of the terminals of the piezoelectric element is connected to ground. Rather, the piezoelectric element is not only able to be charged to a positive voltage, but also to a slightly negative voltage. To this end, one pole of the piezoelectric element (low voltage side) is set to a voltage of approximately 45 V with respect to ground. It is problematic if such a potential of less than 45 V is also applied to a terminal on the high voltage side, since then the differential voltage at the actuator becomes negative. In order to be able to use and monitor such piezoelectric elements, a ground-potential-free measurement of the differential voltage must be provided.